Solar cells have been utilized as a driving energy source in various devices.
Solar cells have a pn junction at their active portion, and silicon is generally used as the semiconductor to constitute such pn junction. It is generally recognized that the use of a single crystal silicon is desirable in view of converting light energy into an electromotive force and that the use of an amorphous silicon is desirable in view of enlarging the area while reducing the production cost.
In recent years, studies have been made of polycrystalline silicons in order to attain reduction in the production cost to the level of that for an amorphous silicon and to attain a high energy conversion efficiency similar to that by a single crystal silicon. None of the methods proposed in the past are sufficient in view of efficient utilization of a material, since a plate-like material is used which is obtained by slicing a polycrystalline lump. It is difficult to make said plate-like material to be less than 0.3 mm in thickness. Because of this, said plate-like material unavoidably becomes thicker than that required for sufficient absorption of light. In other words, it is still necessary to make the corresponding material thinner in order to attain reduction in the production cost.
In view of the above, an attempt has been made to form a polycrystalline silicon thin film by means of a thin film-forming technique such as a chemical vapor deposition process (CVD process). However, the resulting product has a crystal grain size of the order of about 10.sup.-2 microns and is inferior to the resultant obtained by the massive polycrystalline silicon-slicing method in view of energy conversion efficiency.
An attempt has been made to enlarge the crystal grain size of a polycrystalline silicon thin film formed by means of the CVD process by subjecting the thin film to light irradiation to fuse the thin film, followed by recrystallization. However, this attempt is not sufficient to reduce the production cost, wherein it is difficult to attain stable production.
These situations are problematic not only in the case of silicon materials but also in the case of compound semiconductors.
Now, U.S. Pat. No. 4,816,420 discloses a process of producing a thin crystal system solar cell. The solar cell-producing process disclosed in this U.S. patent is characterized by forming a sheet-like crystal on a crystal substrate by selective epitaxial growth and lateral crystal growth through a masking member and removing said crystal from the substrate.
In this method, the exposed portions formed through the masking member are arranged in a line-like state. Removal of the sheet-like crystal grown by selective epitaxial growth and lateral crystal growth depending on the line-like crystal seeds is carried out by mechanically peeling it utilizing the cleavages thereof. However, there is a problem where the crystal seeds in line-like state are above a certain size, the area in contact with the substrate is increased. Because of this, the sheet-like crystal is often damaged during removal thereof. Particularly, in the case of enlarging the area of a solar cell, how much the line width should be narrowed (practically, about 1 .mu.m), a serious problem arises in the case where the line length is of an extent of some millimeters to some centimeters or above.
The above U.S. patent describes an example wherein SiO.sub.2 is used as the masking member and a silicon thin film is grown by way of selective epitaxial growth and lateral crystal growth at a substrate temperature of 1000.degree. C.
It is known that at such a high substrate temperature, a grown silicon layer reacts with SiO.sub.2 to introduce remarkable stacking faults (plane defects) into the silicon layer side in the vicinity of the interface between the silicon layer and the SiO.sub.2. These defects impart seriously negative influences to the characteristics of a solar cell obtained.
On the other hand, it is reported that the defect density of the film is reduced as the film-growing temperature is lowered. (see, H. Kitajima, A. Ishitani, N. Endo and K. Tanno, Jpn. J. Appl. Phys. 22, L783 (1983))
In this case, however it is extremely difficult to attain the film thickness required for the formation of a solar cell since the film-growing speed is markedly reduced.